CT scanners of the third generation type generally comprise a source of X-rays and a detector array, both mounted on a rotatable disk or platform at diametrically opposite sides of the opening through which the scanned object is placed. During a scan the source and detectors rotate about the rotation axis, usually referred to as the Z-axis; and at precise angles of the source and detectors, data values are acquired. The data values are representative of the X-ray photons generated by the source and projected onto and sensed by the detectors at each angle so as to provide projection views corresponding to these angles of view. Some of the X-ray photons are absorbed by the object and the data values are thus a function of the integral of the density of the portion of the object through which the measured X-rays pass for each projection view measuring interval, i.e., the smaller the reading the more X-ray photons absorbed during that time interval, and thus the integral of the mass is more dense along the X-ray path. The field of view or reconstruction circle (i.e., the spatial area through which X-rays measurements are made) is typically defined by the beam angle (the angle of divergence of the beam as it is projected from a focal spot of the X-ray source to the detectors), and the distance the focal spot of the source is placed from the isocenter (the mechanical center of rotation) of the machine.
The detector array is traditionally constructed so that the detectors lie on the circumference of a circle having a center of curvature at the location where the X-rays emerge from the source, i.e., the focal spot, so that the radiation path from the source to each detector is the same, although other geometric arrangements and configurations have been suggested. See, for example, U.S. Pat. No. 5,668,851 entitled X-ray Tomography System With Stabilized Detector Response and issued on Sep. 16, 1997 in the name of John Dobbs; U.S. Pat. No. 5,757,878 entitled Improved Detector Arrangement For X-ray Tomography System and issued on May 26, 1998 in the names of John Dobbs and Ruvin Deych; U.S. Pat. No. 5,781,606 entitled X-ray Tomography System With Substantially Continuous Radiation Detection Zone and issued Jul. 14, 1998 in the names of John Dobbs and Ruvin Deych; and pending U.S. application Ser. No. 08/726638 entitled CT Scanner With Simulated Parallel Beam Design and filed in the United States Patent and Trademark Office on Oct. 7, 1996 in the names of John Dobbs and Ruvin Deych; all of these applications being assigned to the present assignee.
A Z-axis collimator is typically positioned between the X-ray source and the opening of the disk so that the thickness of the fan beam can be controlled in the Z-axis direction and so that the entire beam passing through the field of view (and any object disposed therein) is projected onto the detector array. The thickness of the beam defines the thickness of the slice through the object for which data is acquired. Until fairly recently, the detector array of a CT scanner of the third generation type has consisted of a single row of detectors. In order to generate sufficient data values to reconstruct an image of the portion of the object through which the beam passes, a typical array of the prior art type has had one row of detectors comprising approximately from 300 to 700 detectors, and either 1440 or 2880 projection views have been typically taken for a 360.degree. scan. A machine of such design thus generates 432,000 to 2,016,000 data values per 360.degree. scan. Until recently designing systems to generate more data values per scan was considered cost prohibitive because the detectors have represented a significant portion of the cost of the machine, and limitations have been imposed by the bandwidth of data acquisition systems and reconstruction computing systems for processing the data values.
However, with improvements in increased bandwidth of data acquisition systems and reconstruction computing systems and improvements in detector designs, machines have been developed with two dimensional (2D) detector arrays having multiple rows and columns of detectors. For example, the Elscint Twin CT scanner machine includes a detector array comprising two adjacent rows of detectors. The Z-axis thickness of the X-ray beam is set so that the beam projects onto both rows so that two slices can be simultaneously generated. With improved designs, cone beam systems for generating CT helical scans have become more practical. Such systems utilize a detector array comprising a plurality of rows of detector elements. See, for example, U.S. Pat. No. 5,262,946 entitled Dynamic Volume Scanning for CT Scanners and issued Nov. 16, 1993 to Huescher; U.S. Pat. No. 5,291,402 entitled Helical Scanning Computed Tomography Apparatus and issued on Mar. 1, 1994 to Armin H. Pfoh; U.S. Pat. No. 5,390,226 entitled Method and Apparatus for Pre-Processing Cone Beam Projection Data for Exact Three Dimensional Computer and issued Feb. 14, 1995 to Kwok C. Tam; U.S. Pat. No. 5,510,622 entitled X-ray Detector Array with Reduced Effective Pitch and issued Apr. 23, 1996 to Hui Hu et al; my copending application, now U.S. Pat. No. 5,818,897 entitled Quadrature Transverse CT Detection System and issued Oct. 6, 1998 to Bernard M. Gordon; and EP Published Patent Application, Publication No. EP 715830 published on Jun. 12, 1996, entitled Computerized Tomographic Scanners, and invented by Dale J. Bendula and Heang K. Tuy.
As shown in these references, 2D arrays comprising straight rows and columns of detector elements can be used, such as shown in the Pfoh, Huescher and Gordon patents, and the Bendula publication. Alternative arrangements are suggested in the Hu et al reference, wherein 2D arrays of detector elements are proposed in which the centers of the detector elements are aligned in one direction (either in the direction of the Z-axis or the direction coplanar with or parallel to the X-Y plane of the cone beam) so as to form either a plurality of parallel columns (when aligned in the direction of the Z-axis) as seen in FIGS. 3A, 3C and 3D of the reference; or a plurality of rows (when aligned in the direction of the X-Y planes) as seen in FIG. 3B of the reference. The detector elements however are alternately staggered in the other direction so that their centers are staggered. The Hu et al reference also suggests detector elements shaped as parallelograms so that their centers are aligned along rows and columns in two non-perpendicular directions. Such arrangements are provided to decrease the detector pitch along one or both dimensions of a 2D detector array, which is particularly useful for helical or volumetric scans.
In addition, the Gordon patent describes a 2D detector array comprising modules of two types of rectangularly shaped detector elements, one type having a longer dimension in the Z-axis direction, while the other type having a longer dimension in the direction within the X-Y plane. The latter detector elements are provided to insure detection of thin objects such as sheet explosives oriented parallel to the X-Y plane so as to insure detection.
With the ability to make 2D detector arrays and their use cost effective, machine designs are now being proposed to simultaneously provide multiple slices of the same thickness, and/or provide variable thickness slices. In one proposed design the 2D array comprises relatively small identically sized detector elements, each about 0.5 mm square, and arranged to form a 80 (elements per column) by 896 (elements per row) array. Multiple slices can be simultaneously generated, or variable thickness slices can be selectively generated, by using a corresponding set of select detector elements for each of the slices.
In this regard, a controllable switch is provided at the output of each detector element so that a detector element can be used to acquire data when the switch is on, and ignore any sensed data when the switch is off. All of the outputs of the detector elements of each column are summed together so that when a particular set of rows is switched on, the outputs of the switched detector elements of each column are summed together. In addition, the slice thickness is usually measured at the isocenter with the beam thickness actually being proportionally larger at the detector elements. However, for ease of exposition the slice thickness is described herein as the thickness of the beam portion projected onto the detector array.
Therefore, if a 3 mm slice is desired, the six rows of 0.5 mm square detector elements that are exposed to the beam are switched on, while the remaining elements are switched off. The six switched-on detector elements of each column can then be summed to provide one data value reading for each column for each projection view. Similarly, if multiple slices each of 3 mm are desired, adjacent groups of six rows of detector elements per group are simultaneously used (with the outputs of all of the switched-on detector elements of each column of each group being summed together) for the corresponding portions of the cone beam projected onto the groups of detector elements. In the example, when a 5.0 mm slice is required an adjacent group of ten rows are simultaneously used, with the outputs of the detector elements of each column of the group being switched on and summed together. Thus, any number of slice thicknesses in increments of 0.5 mm, can be provided by choosing the appropriate number of rows of detector elements and summing the outputs of the chosen detector elements, with each row providing an incremental increase of 0.5 mm thickness.
2D arrays comprising these relatively small detector elements, e.g., 0.5 mm, however, exhibit X-ray conversion inefficiencies. More particularly, for slice thicknesses of 1.0 mm and larger, each of the plurality of data values provided at each projection angle is a function of the X-ray photons received and converted by the multiple detector elements constituting a column of switched-on detector elements. Because the efficiency of each detector element typically drops off at its edges, and because no detection occurs in spaces between detector elements, a significant portion of the entire detector area comprising those switched-on detector elements being used to make up each column for receiving X-ray photons does not convert X-ray photons efficiently. Further, image artifacts can result should a detector switch be defective for one or more of the detector elements being used to receive and convert X-ray photons. More specifically, the responses of the columns of switched-on detector elements of each group, connected to provide a summed signal, will not be uniform and thus image artifacts may be created in the reconstructed image.
Accordingly, there is a need for an improved detector arrangement for acquiring CT data for variable thickness slices, and/or for multiple CT slices, and which reduces or overcomes these prior art problems.